The present invention relates to a fuel injection pump for a diesel engine, and more particularly relates to a type of diesel fuel injection pump in which the injection of fuel can be cut off by a fail safe mechanism if abnormally long actual fuel combustion time is detected.
In a diesel engine, the diesel fuel is injected at high pressure by a diesel fuel injection pump through fuel injectors into the cylinders of the engine in turn upon their compression strokes, and ignites due to the natural compression in the cylinders and is combusted therein without any special electrical or mechanical ignition means being required. Therefore in such a diesel engine there is a risk that if the fuel injection pump develops some abnormality the injection of fuel may be performed to too great an extent. For example, the injection of fuel may be performed in an amount corresponding to full engine load, even when the load on the engine is less than full load; or, worse, the injection of fuel may be continued to be performed, even when it is desired to completely terminate fuel injection and to stop the diesel engine running. In such a case, the danger arises of the diesel engine overrunning or overrevving, and this type of malfunction can be very troublesome.
There is known a type of fuel injection pump for a diesel internal combustion engine which includes a plunger which reciprocates to and fro in a bore defined in a housing, a high pressure chamber being defined between one end of the plunger and the end of the bore. During the suction stroke of the plunger as this high pressure chamber expands in size, diesel fuel is sucked into this high pressure chamber from a quantity of diesel fuel contained in a relatively low pressure chamber through a fuel supply passage; and during the compression stroke of the plunger as the high pressure chamber subsequently contacts in size, the diesel fuel in the high pressure chamber is squeezed and is brought to a high pressure and is ejected through an injection passage therefor to a fuel injector of the diesel internal combustion engine. Sometimes, in the case that the diesel fuel injection pump is a so called distribution type pump, the plunger is rotated as it reciprocates by an input shaft which is rotationally coupled to it although not axially coupled to it, and by a per se well known construction the spurt of highly compressed diesel fuel is directed to the appropriate one of the plurality of cylinders of the internal combustion engine. Now, such a fuel injection pump injects an amount of diesel fuel in each pump stroke which is regulated by a fuel injection amount control means which selectively vents the high pressure chamber. This control means ceases to vent the high pressure chamber when it is appropriate to start the fuel injection spirit, during the compression stroke of the plunger, and at this instant the almost incompressible diesel fuel in the high pressure chamber starts to be squeezed and injected, as explained above. When it is appropriate to terminate the fuel injection spirit, then the control means starts again to vent the high pressure chamber, and at this instant the diesel fuel in the high pressure chamber ceases to be squeezed and therefore the injection is immediately stopped.
In the case of a mechanical diesel fuel injection pump, it has been conventional for this high pressure chamber selective venting means to be a spill ring, which is mechanically positioned according to the position of the accelerator pedal which is controlling the load on the engine, and whose position controls the timing instant of the end of the non-vented time period of the high pressure chamber. In such a mechanical type of fuel injection diesel pump, it is very rare for such a malfunction to develop as that the venting of the high pressure chamber should fail, because of the simple structure of the spill ring, and because of the fact that tyically the accelerator pedal simply positions the spill ring through a simple linkage, and in such a construction there is no requirement for a system to prevent engine overrunning of the sort described above. However, in a more sophisticated mechanical type system of this sort, in which the linkage between the position of the accelerator pedal and the position of the spill ring is not a simple mechanical one but is, for example, performed electronically, it has been known to compare the required fuel injection amount with the actual position of the spill ring and to interrupt fuel injection if they do not agree, at least to within some prescribed margin of error.
However, nowadays electronically controlled fuel injection pumps are coming into use, in which the selective venting of the high pressure chamber is performed, not mechanically by the use of a spill ring, electronically by an electromagnetic valve which is controlled by an electronic control system such as one incorporating a microcomputer. In such an electronic fuel injection pump, the electronic control system, for each spirit of fuel injection, calculates how much fuel is to be injected in this spirit, and then at an appropriate time point for the start of fuel injection closes said electromagnetic valve, so as to terminate fuel spilling from the high pressure chamber and so as thereby to start fuel injection. After the electronic control system has calculated that the proper amount of fuel has been injected by the movement of the plunger in the direction to reduce the size of the high pressure chamber, then said control system opens said electromagnetic valve for fuel spilling again, thus immediately terminating fuel injection. In such an electronic type of fuel injection pump, since there exists no spill ring, such a comparison of the required fuel injection amount with the actual position of the spill ring is of course impossible, and accordingly some other method is required for preventing engine overrunning. Further, because of the electronic nature of the fuel injection control system, the requirement for a means for preventing engine overrunning is very much more critical than in the case of a mechanically regulated fuel injection pump, because no simple and reliable mechanical linkage can be relied upon for reducing fuel injection amount when engine load is to be reduced, and there is a very real risk that malfunctioning of the control system for the electromagnetic valve for fuel spilling could cause very much too great fuel injection amount, possibly leading to running away of the diesel engine. Similarly, in the case of control system malfunction, in the absence of any means for preventing engine overrunning, there would be a great risk that when it was desired to stop the diesel engine operating it would be impossible to do so, and since there is not available any way of interrupting ignition of the fuel in the engine cylinders such as in a conventional electrically ignition gasoline engine, great trouble could thereby be caused.